1. Field of the Invention
The present invention generally relates to voltage references. More particularly, the present invention relates to autocalibrated voltage references which improves long term drift performance and optimizes reference performance characteristics without compromising reference voltage temperature drift and accuracy.
2. Description of the Related Art
The design of bandgap voltage references generally entails a series of trade offs of several different characteristics of the voltage references against accuracy and temperature drift. Often, the design of a voltage reference is optimized to consume the minimum amount of power supply current. With this optimization goal, however, it is difficult to maintain low temperature drift. The same is true when the design of the voltage reference is optimized for output voltage noise. In the latter case of output voltage noise, the core elements of the bandgap voltage reference is the major source of the output voltage noise. Thus, bandgap voltage references are not well suited for applications particularly sensitive to high levels of noise.
Other approaches, including operating metal oxide semiconductor (MOS) devices in the sub-threshold region to create a bipolar-like bandgap voltage reference, have higher temperature drift. On the other hand, while a buried zener does not exhibit a high temperature drift, it is not suited for a power supply of less than 5 volts. This is due to the fact that the normal operating voltage of a P-N junction operating in the zener breakdown region is approximately 6.5 volts. In addition, diodes are stacked on top of a zener such that the combined temperature coefficient is close to zero. Moreover, buried zener references commonly require a power supply in excess of 12 volts.
The change in semiconductor components, otherwise known as long term electronic ("voltage") drift, occurs during normal operation. This is a known accuracy limitation for a MOS voltage reference. For example, the gate-to-source voltage V.sub.gs, of an N-channel metal-oxide semiconductor field effect transistor (MOSFET) can change as much as 10mV for the same drain current I.sub.d over a 1000 hour HTB reliability test. For MOSFETs, this drift is predominantly due to charge accumulation in the gate oxide as well as due to changes in the surface/oxide interface charge. The changes to the surface/oxide interface charge do not occur without power being applied to the gate and drain terminals of the MOSFET. Furthermore, measurements indicate that a MOS device with no power applied thereto does not experience voltage drift other than those shifts in threshold voltage and .beta. (beta) that are due to relaxation from packaging stress. In turn, this packaging stress can be significantly reduced by several unpowered post-packaging temperature cycles prior to initial testing and calibration.
Even when the voltage reference is trimmed at the time of manufacturing to reduce or minimize the error from the desired ideal voltage, its output voltage drift as a function of time (referred to as long term stability) is approximately 100 parts per million (PPM) per volt for 1000 hours of operation over the operating life of the voltage reference.